In the front-end of terminal devices for mobile communication, for example, in cellular telephones, SAW filters (SAW=surface acoustic wave) are primarily used today as band-pass filters in the HF band. These are, in the main, constructed as reactance or DMS filters (DMS=double mode SAW).
The filters that are to be used in terminal devices for mobile communication should have as small an insertion attenuation of the useful signal as possible. The use of modern modulation methods requires higher selection and opposite-sideband suppression from the filters used in the sending and receiving path, as compared to traditional SAW filters.
There are, for example, circuits made by connecting DMS filters with reactance elements, in particular one-port resonators constructed with the SAW technology that are characterized by slight insertion attenuation. For example, publication DE 198 18 038 A describes a DMS filter in which two DMS filters connected in series or parallel are connected in series with reactance elements at the entrance or the exit side.
Publication DE 100 07 178 A1 describes a DMS filter that is connected with a two-port resonator with two acoustically coupled serial transducers, each of the serial transducers being connected in series with one of the transducers of the DMS filter. In that configuration, the series branches of the circuit are connected between a symmetrically constructed entrance and a symmetrically constructed exit.